one possible interpretation is that you want coverage for all the
packages involved in the fmt test, not all the packages in the world.
Because coverpkg was previously defined as a list of packages
to be loaded, however, it meant all packages in the world.
Now that the go command has a concept of package notation
being used as a matching filter instead of a direct enumeration,
apply that to -coverpkg, so that -coverpkg=all now has the
more useful filter interpretation.